The programmed cell death known as apoptosis is distinct from cell death due to necrosis. Apoptosis occurs in embryogenesis, metamorphosis, endocrine-dependent tissue atrophy, normal tissue turnover, and death of immune thymocytes (induced through their antigen-receptor complex or by glucocorticoids) (Itoh et al., Cell 66:233, 1991). During maturation of T-cells in the thymus, T-cells that recognize self-antigens are destroyed through the apoptotic process, whereas others are positively selected. The possibility that some T-cells recognizing certain self epitopes (e.g., inefficiently processed and presented antigenic determinants of a given self protein) escape this elimination process and subsequently play a role in autoimmune diseases has been suggested (Gammon et al., Immunology Today 12:193, 1991).
A cell surface antigen known as Fas has been reported to mediate apoptosis and is believed to play a role in clonal deletion of self-reactive T-cells (Itoh et al., Cell 66:233, 1991; Watanabe-Fukunage et al., Nature 356:314, 1992). Cross-linking a specific monoclonal antibody to Fas has been reported to induce various cell lines to undergo apoptosis (Yonehara et al., J. Exp. Med., 169:1747, 1989; Trauth et al., Science, 245:301, 1989). However, under certain conditions, binding of a specific monoclonal antibody to Fas can have a costimulatory effect on freshly isolated T cells (Alderson et al., J. Exp. Med. 178:2231, 1993).
DNAs encoding a rat Fas ligand (Suda et al., Cell, 75:1169, 1993) and a human Fas ligand (Takahashi et al., International Immunology 6:1567, 1994) have been isolated. Binding of the Fas ligand to cells expressing Fas antigen has been demonstrated to induce apoptosis (Suda et al., supra, and Takahashi et al., supra).
Investigation into the existence and identity of other molecule(s) that play a role in apoptosis is desirable. Identifying such molecules would provide an additional means of regulating apoptosis, as well as providing further insight into the development of self-tolerance by the immune system and the etiology of autoimmune diseases.